When does each trimester begin?

Can you please clarify when the first, second and third trimesters begin?

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Peg Plumbo CNM

Peg Plumbo has been a certified nurse-midwife (CNM) since 1976. She has assisted at over 1,000 births and currently teaches in the... Read more

By definition, the word "trimester" divides a time into three equal parts. However, pregnancy is not neatly divisible by three. If you calculate from the last menstrual period (LMP) with a 28 day cycle, a pregnancy lasts 40 weeks or 10 lunar months. If you calculate from probable conception, again based on a 28 day cycle, you are left with a gestational period of 38 weeks. Neither of these are divisible by three. As a result, you get somewhere between 12 2/3 weeks and 13 1/3 weeks.

By convention, it is common to say that the first trimester ends at 14 weeks (14 weeks from the first day of the last period). The second trimester proceeds from 14 to 28 weeks and the third trimester extends from 28 to 40 weeks.

I suppose it's our quest for symmetry that requires that we break time periods into a beginning, middle and end. It also gives care providers neat compartments in which to discuss changes in physiology, nutritional needs, common discomforts and more.

Otherwise, there is no real significance to dividing the pregnancy into three equal parts. Care providers tend to speak of weeks (from last menses) rather than trimesters.

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